1. Field of the Invention
The invention relates to fabrication of silica optical fiber, in particular fabrication of fiber preforms by a rod-in-tube technique.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform. The preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overdadding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816, the disclosures of which are hereby incorporated by reference. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform, as discussed in U.S. Pat. No. 4,775,401, the disclosure of which is hereby incorporated by reference.
Forming a fiber preform using such a process therefore requires both a substrate tube and an overcladding tube. Previously, both types of tubes were formed from fused quartz or by soot methods, i.e., depositing glass on a mandrel by directing at the mandrel glass particles formed by flame hydrolysis of silicon tetrachloride. Both methods were energy intensive and costly, however, and alternatives were sought.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass does not have to meet the optical performance specifications to which the core and the inner cladding must conform (but still must be substantially free of flaw-inducing refractory oxide particles). For this reason, efforts to both ease and speed manufacture of fiber preforms focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process.
U.S. Pat. No. 5,240,488 (the ""488 patent), the disclosure of which is hereby incorporated by reference, discloses a sol-gel casting process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In the process of the ""488 patent, a colloidal silicon dispersion, e.g., filmed silica, is obtained. To maintain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 11 to about 14 by use of a base, and the dispersion is then aged. Subsequent to aging, as discussed in Col. 15, lines 39-65 of the ""488 patent, a gelling agent such as methyl formate is added to the dispersion to lower the pH. Typically, once the gelling agent is added, but before gellation occurs, the mixture is pumped into a tubular mold containing a central mandrel, and the gel is aged in the mold for 1 to 24 hours. The mandrel is removed, and the gelled body is then extracted from the mold, typically by launching the body from the mold in water to prevent breakage. The body is then dried, fired to remove volatile organic materials and water, and then sintered to form the finished overcladding tube. The core rod is then inserted into the tube, and the assembly is collapsed into a fiber preform.
There are numerous issues involved in performing, for example, the steps of drying, firing, and sintering, since the intent is to produce a tube meeting relatively stringent physical and chemical specifications. One such specification is the bow of the resultant overcladding tube. The bow is desirably as small as possible, since any bow present can potentially interfere with core rod insertion, which can ultimately affect the properties of the fiber drawn from the finished preform. Techniques for attaining overcladding tubes having very little bow are therefore desired.
The invention involves fabricating a ceramic tube, e.g., a silica overcladding tube, with very little bow, e.g., as low as 0.3 mm per meter. In particular, the invention involves securing the tube by a handle that both allows the tube to hang plumb during treatment, and also is capable of deforming during such heat treatment to maintain the plumb arrangement. In one embodiment, reflected in FIG. 1C, a handle tip 22 is inserted into the bore of the tube 10 such that holes 12, 24 in the tube and the tip are aligned, and a silica pin 30 is inserted. The tube is hung by securing the opposing end of the handle, and the tube/handle assembly is fired at a temperature suitable to remove any remaining impurities, generally by raising the assembly through the hot zone of a furnace. Once the entire tube has been fired, the furnace is generally heated to a sintering temperature, and the tube is pulled back up through the hot zone (an ascending sinter). The green silica tube tends to shrink to xc2xe of its original diameter during this sinter. However, the high temperature softens the silica and thereby induces some flexibility to the tube and the handle tip. The tube thus deforms around the tip, forming a continuous interface, with the tip deforming as well. The handle tip tends to deform in the plumb arrangement, as opposed to the tube being forced to bow to remain plumb, and this deformation of the tip substantially reduces inducement of bow in the tube.